sparc: simplify syscall_get_arch()

Include linux/thread_info.h so we can use is_32_bit_task() cleanly.
Then just simplify syscall_get_arch() since is_32_bit_task() works for
all configuration options.

Suggested-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Eric Paris <eparis@redhat.com>